ISHS Acta Horticulturae 170: Greenhouse Construction and Covering Materials
A. A STUDY OF GREENHOUSE FOUNDATIONS - NEW SYSTEMS |
| Author: | Ing. J.C. Spek |
| Abstract:
The Dutch standard for greenhouses NEN 3859 specifies that all vertical and horizontal forces acting on the greenhouse must be transferred to the ground via the foundations. For example, with greenhouses having 6.4 m lattice girders (2 bays)* this is done by drilling a hole into the ground and pouring into it an in-situ cast concrete base block 400 mm below ground level. This then combines with the undisturbed soil: shear stresses and skin friction are also found to be present in tests and thus can be taken into account. A prefabricated reinforced concrete pile is placed into the concrete base and extends about 300 mm above ground. The pile is 120 x 120 x 1000 mm long. The stanchion of the house is then attached to the head of the pile to resist displacement and uplift. The attachment must act as a hinge joint. The reinforcement consists of four bars. On the basis of information from soil mechanics and practical tests, the size of the base blocks has been calculated with the aid of a computer program for three types of soil and four different widths of the greenhouses, having variable numbers of connected spans, different eave heights and three types of piles (gable pile, pile for the bracing bars, central pile). 0.5 kgf cm2 is chosen as the permissible earth pressure for sustained loads. In 1982 a large number of concrete piles of the foundation type, especially those from the 1976–79 period, were found to be attacked by sulphates. This applies to something of the order of 80 000 – 200 000 piles. At a size of compartment of 3 x 6.4 each pile supports 19.2 m2 of greenhouse. This means that it involves about 1 600 000 – 4 000 000 m2 of greenhouses out of a total area of 9 000 ha = 90 000 000 m2. This sulphate attack, which leads to disintegration of the concrete, starts in the core of the pile and causes swelling (figure la). This swelling first becomes visible in the form of small cracks on the surface of the concrete pile above ground, but is also audible when the pile is knocked gently. A pile thus attacked loses all connection over a considerable stretch (figure lb) and is, therefore, no longer able to function properly under load, particularly in the presence of gusts of gales. In August 1984 study has started on:
A report on these problems can be expected in October, after which investigations are likely to follow. | |
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